I honestly don't care enough. It's not a big thing for me.
She was civil with him in "Becoming" but nothing more. It seems too convenient that he would happen by Buffy's house on his way to town and decide to drop by for a chat. Not impossible, but unlikely.
Consider it from Spike's perspective and the things he's been doing all episode; he's wandering around the places he's been before, he's heartbroken and he has a thing for Mum-figures.
From his perspective he got on alright with Joyce- she was a surprisingly polite hostess give what was happening- and he wants to vent to anybody willing to listen; Willow, Joyce, Buffy and Angel, anybody.
As for the rest there are already a whole bunch of coincidences of timing involved in this episode; Spike going to the school and finding the exact lab where Willow and Xander are, Angel dropping by Buffy's kitchen window just at the right time to notice Spike with Joyce. Then Buffy arriving mere moments later.
I'm gonna need more than TV coincidence to believe that Willow would do something pretty unwise, that Spike would be dumb enough to fall for it and then for none of it to be referenced. Surely Willow would've told Xander
something if she'd sent Spike to Buffy's house? Some glimmer of hope for him? But she didn't. It's not inconceivable that Willow was too terrified to think up any cunning plan and just told him to pick up more stuff.
...In fact, doesn't Spike have, like, a list of ingredients in his pocket?
YES! Just checked the script and Spike has a list of spell ingredients in his pocket. Why would Willow give him spell ingredients if she was just gonna send him to Buffy's to pick up her stuff?
She didn't. She sent him to the Magic Box for brand new stuff. That's the only thing that makes sense.
It's hardly Willow's fault if Spike kills someone.
Not fault exactly, but she would've been culpable of doing something unwise and putting others in danger.
Some on this thread are already saying that what Willow did was wrong, even when there's zero evidence that she did anything.